1. Field of the Invention
The present invention relates generally to Coriolis type mass flowmeters and more particularly to an improved mass flowmeter having a flow-sensing structure in which both driving and responsive vibrations are experienced in a plane defined by the looped structure.
2. Description of the Prior Art
Most existing Coriolis mass flowmeters utilize flow-sensing structures including two tubes, or tube segments, with flow directed through them in a hydraulically serial or "parallel" manner. The use of two separate tubes is the favored configuration in that it allows tuning-fork-like vibrational modes as a means of cancelling out common mode vibrations and extraneous noise. However, the use of two tubes and their associated manifolding and/or flow-splitting parts normally make the device expensive, and the presence of flow-splitting or manifolding castings may make it extremely difficult to meet sanitary/cleanability requirements for interior surface finish.
Many dual-tube Coriolis flowmeters are also not self-draining and are thus not suitable for applications where high-level sanitation or contamination of residual material is of concern. Moreover, dual tube Coriolis meters that split the flow often experience clogging in one of the tubes which can go undetected and give rise to measurement error in addition to sanitary problems.
One approach to solving the sanitary problem is disclosed in Levien et al., U.S. Pat. No. 4,730,501, entitled "Single Tube Parallel Flow Coriolis Mass Flow Sensor". In this device, elongated crossing portions of a single length of looped conduit form parallel sensor segments, the ends of which are defined by isolation plates secured to corresponding opposite ends of the segments. While this approach clearly solves the sanitary and cleanliness problem, it has the disadvantage that, since both ends of the active tube segments are normally fixedly supported, fluid temperature changes give rise to axial stresses that adversely effect meter performance.
Some Coriolis flowmeters have flexible joints, couplings or conduit loops that render them susceptible to pressure changes, sanitary, cleanability and drainability problems. One example of a relevant device having a flexible intermediate section coupling an active inlet segment to an active outlet segment is disclosed in the U.S. Pat. No. 4,811,606, to Hasegawa et al., entitled "Mass Flowmeter". In such apparatus the straight tubular segments are fixed at one end and swung by a vibrator at the opposite end, such that the distal ends of the tubes alternately approach and depart to and from each other. The ends of the tubes subjected to vibration are connected by a flexible tube. However, it is now understood that flow measurement errors can result from the use of such a flexible tube in the following ways:
1) Vibrational drive motion can be damped by a flexible section thereby causing variations in "drive" excitation. Damping can also compromise (i.e. dampen) the responsiveness of such a structure to Coriolis forces. PA1 2) Pressure changes can introduce unwanted motions/distortions into the flow tube assembly (and hence flow measurement errors) due to the adverse pressure sensitivity of a flexible intermediate section whose flexibility arises from a conduit section that is deformed or shaped in the form of a loop or a bellows-like structure or from another flexible material such as rubber or plastic. Hydrostatic pressure changes can cause variations in the elasticity of a flexible section, further complicating the ability to obtain accurate flow measurements. PA1 3) In addition to the pressure sensitivities of a flexible section, the temperature variation of its elastic modulus (in addition to that of the inlet/outlet sections) may contribute additional flow measurement errors and other errors from variable zero flow offsets. PA1 4) Flexible sections made of an elastic material or bellows may not meet the "sanitary" or cleanability requirements of many food, pharmaceutical, beverage or chemical applications, nor will a flexible section comprised of loops necessarily be self-draining. PA1 5) A flexible section fabricated from bent conduit and looped in some manner will also present a higher pressure drop to the flowing fluid due to the additional number of turns presented to the flowing fluid, thereby further restricting the usable flow range of the meter. PA1 6) The use of a flexible intermediate section may require that the flexible section be attached or somehow secured to the support. If so, then additional and unwanted stresses may be introduced into the flow tube structure.